Electrical pulse secrecy communication system



E. M. DELORATNE ErAL 2,582,968

ELECTRICAL PULSE sECRECY COMMUNICATION SYSTEM Jan. 22, 1952 5 Sheets-Sheet 1 Criginal Filed Deo. 51, 1941 Jan.

Original Filed Dec. (51,.1941

E. M. DELORAINE len-A1. 2,582,968

ELECTRICAL PULSE SECRECY COMMUNICATION SYSTEM 5 Sheets-Sheet 2 ATTORNEY.

5 Sheets-Sheet 5 INVENTORS Jan. 22, 1952 E. M. DELoRAlNE ErAL ELECTRICAL PULSE sECRIxCY COMMUNICATION SYSTEM Original Filed Dec. 31, 1941 E. M. DELORAINE Erm.

ELECTRICAL PULSE SECRECY COMMUNICATION SYSTEM original Filed Dec. 31. 1941 Jan. 2.2, 1952 5 sheets-sheet 4 W E i.

w Max `Fam. 22, i952 E. M. DELORAINE ETAL 2,582,968

ELECTRICAL PULSE sECRECY COMMUNICATION SYSTEM 5 Sheets-Sheet 5 Original Filed Dec. 5l, 1941 Patented Jan. 22, 1932 2.582,9@ LoiitIo-L njs'E SEECBM OATIGN SYSTEM :Edmond -M. "Deloraine, Paris', France, andlilmile `Labin. Newlnrk. N- Y, assignee t0 :Federal ,'.lelephone end lalio Corporation, .New York, N. Y., a corporation ofvDelawae fowl-tinnen@ @1f-application :serial JN0. Y425,108. December 131', .19421. Thisfa'pplieation `October V10; .1945, SerialiN o.-.621,584f

in eis-nas". (ci. 25o-6) The present invention Arelates to systeisof communication and more particularly'to signaling systems providing a'measure ofsecreey.

The present invention is acontinuation of our application Ser. No. 425.108, -filed December 3l, 1941, now abandoned, and -is closely related to and in some of its aspects constitutes yan vim4 provement over the copending application of Emile Labin, iiledApril 1,. 1-941`Serial.No.386.282, now U. S. Patent No. `2,406,0l9iv granted .August 20, 1946, relating to Pulse Modulation Systems.`

One object of the invention is to guard the secrecy of a message whiohistransmitted b yone method, e. g., 'by some uncommon modulation method, by simultaneously sending a 4masking message by another method, e. g., by a more usual; form of modula-tion. The masking message conveys information which is unimportan-tandits detection by an unauthorized person; will aidin the concealment of thesecret-message.

More speci-ficallmit is an `vobject of the present invention toA provide `a signaling system wherein secret signals which are conveyed by time modulation of pulses are screened from observation by modulating the amplitude of thefpulses and thus sending amplitude .-modulated-sigxials'on the same channel.

According to another feature, signal .receiversare arranged to receive intelligibly the .messages in spite-of the simultaneous reception of-'messagesv and jamming signals.

Another object of the invention is a .receiver responsive to signals whose amplitudefand time positions are modulated, and arranged to reproduce the amplitude modulationof thesignalsand separately to reproduce the time modulation of the signals.

In the preferred embodiment of our invention,- the masking message and the :masked secret message a-re used to amplitude and time modulate the same pulses. However, `it ispwithin the-scope of our invention to provide additional pulsesinterspersed between the pulses which `are-time modulated by the secret signal, and' to :modulate the amplitude or timing or both, of saidadditional pulses With the unimportant maskingsignal.

According to a featurefof the present invention-,v

a signaling system comprises a jamming transmitter and one or more message signaltransmit-- ters and receivers of the above mentioned type. The jamming transmitter .and Iat least `one .of the signal transmitters are remote Afrom one .another but are synchronized. i A *Y Preferably, the desired timing of' a signal trans'- 2 Ystation with respect to a remotefjamm'ifng station is accomplished bymeans provided at the signal transmitting station. However, the relationshipmay be reversed and the timing means provi-dedatthe jamming station.

|Ihe simultaneous operation of a .jamming and a signal transmitting station is useful no'tro'ly defensively for enhancing theA secrecy of the transmitted signals, butv also offensively 'for interfering with communications `of an enemy. `./.Ltransmitter -as well as a combination transmitter and receiver capable of performing vs orne or all of the above mentioned functions .form separate objects-of the invention. n .'Ihes'eV and other objects of our invention .may best be understood 'from theappended claims and .tliefo'lloWing description of certain embodiments thereofv as'V applied to the drawings,I .in Wlfh'r Y A Fig'. 1 -represents a combined signal transmitting andreceiving station in accordance with our invention; Y j l Y* Fig. 2- represents a jamming and monitoring station-:for usein the systemp n g u Y `Fig. 3 is a detailed schematic circuit of theconverter unit. show-ri in Fig. l; y l

'Fig. 2lrvepresen'ts another form ofl combined signal ltransnitti'ngA and receivingl station which may' 5e l'l'sed.V in place of the embodiment Shown inFig. 1;

l'igsl 5 andi'" are curvesl useful in describingV th'eopration of the station'sshovvn inFigs. 'l and 'Referring more specifically to Fig. 1, 'I a stalle's'oureof sine Waves of a frequency high cor'rparison with 'the' highest signal Vfrequency to beftransiiii't'ed'. Thus. if s'i'gna'lsto be'Y transmitted are lovv quality speech signals in the range oi 6D'-'3`0`00'cyclespersecond. the Wave source l may beiastable osoillatorof' k`c'. frequency.

.Elements '2, 3 and'l'ilare respectively a fu11 ivave rectifier, -a diierentiating circuit such as anconde'nserre's'istance circuit` andanother diier'entiating circuit which may be the same as, 3 orV limiter circuit l .provided which, .infwell 'lnow fashion, passes only s`o` much of' the applied'signalY as lies between two fixed levels of amplitude. Such level limiters are well known and may, for example, be constituted by a vacuum tube biased below cut-off (whereby signals weaker than a given lower level are eliminated), and having the input signal applied to its grid through a high impedance (whereby all signals having an amplitude greater than that required for drawing grid currents are cut off). Preferably, the twolevels of limiter 4 are chosen as shown by dot-anddash lines :v and y in curve K of Fig. 6, so that a large part of the positive portion of the output of diierentiating circuit 3 is transmitted through the limiter 4, but none of the negative portion.

The sharp pulse output of the second differentiating circuit 5 is, together with two other impulses, applied to a converter unit I0, hereafter more fully described.

Part of the energy from the output of the second differentiating circuit 5 is applied to a control element 6. This element E is essentially a multi-vibrator adjusted to operate at a frequency somewhat below 6 kcrwith substantially equal times of dwell in both its conditions of quasi-stability. The input circuit to such multivibrator is arranged in known marmer so that a pulse of a given polarity (say positive), is K capable of triggering the multi-vibrator from a condition B to a condition A, if the multi-vibrator has for some time been in condition B and is almost ready spontaneously to shift over to condition A.

Multi-vibrators having their input circuits so arranged are known and are usually designated as doubly triggered or doubly controlled multivibrators. The output of multi-vibrator 6 passes to another two-level limiter 1 whereby the sloping tops of the positive and negative saw teeth which constitute the output of 6 are flattened out so as to produce almost perfectly rectangular wave form. The square wave output from 'l is applied through an artificial line 9 to converter unit Il).

The third input of converter unitV I0 comesV from a speech signal source 8, such as a telephone which, in the embodiment shown, produces speech signals of low fidelity lying between the frequency limits of 60 and 3000 c. p. s., the signals from source 8 being the secret signals whose very existence it is desired to conceal.

The converter unit I0 to which are applied the needle-like signals from 5, the square block signals from 'I and the speech signals from 8, is a circuit serving to convert the amplitude changes of speech signals 8 into time modulation of the needle sharp pulses from 5. Such circuit is disclosed in the copending application of Emile Labin above referred to and is illustrated in detail in Fig. 3. Essentially it consists of a push-pull amplifier comprising two tubes 62 and 63, and a doubly triggered multi-vibrator comprising two tubes 80 and 90 and controlled by the output of said push-pull amplifiers 62, 63. The details of application of the signal to the push-pull amplifier 62, 63, as well as the details of adjustment of the multi-vibrators 80 and 90 will be described in connection with the y operation of the system. Y

The converter unit I0 performs the function of time modulating the pulses received from circuit 5 in accordance with the speech or other message from source 8 whose secrecy it is desired to preserve. The output of converter I0 consists of a series of sharp pulses which are normally unequally spaced and which become more unequally spaced in response to positive signal amplitudes from 8 and on the other hand tend to become more equally spaced in response to negative signal amplitudes from 8. In other words, the successive'pulses are displaced toward and away from each other in push-pull according to the instantaneous values of signal energy. A two-level limiter II receives the output of said converter I0 and serves to render the needlelike pulses of constant amplitude so that only the time position variations of these pulses are passed to the output of I I.

Element I2 is another source of low fidelity speech or other message signals of 60-3000 c. p. s., such as a'telephone or microphone. This is the source of the unimportant or message masking the secret message originating in source 8. The

variable amplitude output of source I2 is combined with the constant amplitude time modulated needle-like pulses from limiter I I in a combining unit I3. This may be an ordinary modulator arranged to modulate the signals from II in accordance with the amplitude of the signals from I2. The output of combining unit I3 consists of a number of needle-like pulses whose time position is the same as in the output oi converter I0 and limiter II. but whose amplitudes correspond to the amplitudes of the speech signals from I2.

The carrier frequency oscillator I5 is modulated in known manner by the signals from combining unit I3 and yields a brief train of carrier frequency waves for each needle-like pulse in the output of combiner I3. The combiner I3 delivers about 12,000 sharp pulses in one second and, preferably, the duration of each pulse is of the order of 1 to 10 microseconds. Such signals could suitably be carried on an ultra-high carrier frequency of 50 to 500 megacycles.

The output of oscillator I5 consists of a number of trains vof carrier waves, of say 300 megacycles frequency, each train being 600 wavelengths long (i. e. 2 microseconds in duration). The different trains are spaced roughly 11g of one hundredth of a second apart in time but the spacings between successive waves are alternately larger and smaller than this, the difference between two adjacent spacings being normally about 4 microseconds, increased by as much as Smicroseconds during intervals of maximum positive signal from source 8 and decreased toward zero during intervals of maximum negative signal from source 8.

In addition to the variation in spacing of the successive wave trains, their amplitudes are also varied in accordance with the momentary amplitudes of the signals from source I2. The customary receiver will readily detect the amplitude modulation which corresponds to the signals from I2 but will not the time duration signals corresponding to the speech from source 8. Moreover, it will not ordinarily be suspected that there are any other signals than the amplitude modulated signals corresponding to the speech from source I2.

The time and amplitude modul-ated briei` trains of carrier waves from oscillator I5 are delivered over conjugate coupling network I6 to a, preferably, directive antenna I8. Impedance I1 represents a balancing impedance or artificial line or the like, simulating antenna I 8. The conjugate coupling network I6 is of any known type which preserves conjugacy between one pair of terminals the leads from the output of oscilla- `tor I5) and another pair of terminals (the leads of the secret message is doubly screened; it is in the rst place concealed by the jamming signals from .therjamming station, and second, it is concealed by the above described amplitude modulation Vof the signaling wave itself in accordance with signals from source l2.

Since System A is one of the simplestfembodiments of the present invention, it will first be described.

OPERATION OF SYSTEM A Transmission of signal pulses in the absenceof jamming pulses The transmission of a message from one combined transmitting and receiving station, such as illustrated in Fig. l -toanother similar station may take place either in the presence or absence of jamming signals. yAssume first that secret signals are transmitted from one combined station to the other when the jamming station is not in operation.

The station desiring to transmit the message will have, as shown in Fig. l, its talk-listen switch and its jam/no jam switch to the left. The operation under these conditions has already been described. Here we shall explain only the converter in greater detail.

Referring to Fig. 3, the rectangular waves from l are applied to vacuum tubes 62 and 63 in such a way as effectively to undergo full wave rectification. The tubes are arranged push-pull, are biased beyond cut-olf and their output is taken oi in parallel. If the input lwere applied perfectly symmetrically and the two tubes were identical, then symmetrical full wave rectification of the rectangular wave input would result. The input is applied through transformer 6i) whose center tap is constituted by a tap on the potentiometer 6|. By adjusting this center tap" slightly off center, the input of tubes 62 and 63 may be unbalanced, producing in their output the wave formI illustrated at the left of curve A of Fig. 5. This may be called a compromise between full wave and half wave rectification of a rectangular wave form. By virtue of the full wave redtifying characteristics, both the flat topped positive peaks and the fiat topped negative variations of the rectangular input wave appear as negative voltages in the output of tubes 62 and 63.

conversions (say the conversion of the positive peaks) is more efcient than the other and, therefore, the positive peaks yield larger negative voltages than the negative valleys yield positive voltages.

The above explanation assumes that the only unbalance in the amplifier 62, 63 is that introduced by the setting of potentiometer 6I. Actually, however, a separate source of unbalance is introduced by the application of speech signals via transformer 69 to the inner grids of tubes 62 and 63. Such speech signals tend to increase the current through tube 63 and decrease that through tube 62 which, in turn, will increase the degree of unbalance of the output. The degree of unbalance of the resulting waves will vary in dependence upon the magnitude and polarity of the speech signals arriving over transformer 69.- When these speech signals attain their maximum positive value, the unbalance will be greatest' and the output waves will resemble the left curve A, Fig. 5.

When'the speech signals attain their maximum negative value, the imbalance bias will just neu- However, on account of the unbalanced condition of the rectifier, one of these tralize the unbalance in the output caused by the asymmetry of potentiometer 6l. Assuming perfectly balanced full wave rectification of the rectangular waves applied through transformer 60, the output will be as shown in the right half of curve A, Fig. 5.

In addition to the rectangular waves and speech waves, the combined effects of which are illustrated in the left and right hand portions of curve A, Fig. 5, there is also applied to the tubes 62 and 63 a series of needle sharp pulses from differentiating circuit 5, Fig. 1. All this will aiect the output of tubes 62 and 63 as illustrated in curve B. Fig. 5. This left hand portion of this curve represents the output when speech signals over transformer 69 are maximum, and the right hand portion represents the output of tubes 62 and 63 when said speech signals have their most negative value. Since the amplitude of the pulses applied from circuit v5 may be rather high, the control grids of tubes 62 and 63 to which these pulses are applied are negatively biased from battery Il through high resistance 'l0 and potentiometer l2 which is passed by condenser 73. This battery bias is in addition to the negative self-bias of all the grids of these tubes owing to the provision of cathode resistors 64 and 68 which are bj1-passed by the condensers 65 and 61.

The output of tubes 62 and 63 is applied over the resistance andcondenser coupling arrangement 14, T5, 1G, to the control grid of a vacuum tube 85.. Tube 8i! is so connected with another vacuum tube 90 as to form a signal triggered multi-vibrator, i. e., one which may be operated to control the transition from one condition to another, but which operates spontaneously without external controll to re-establish the original condition. The multi-vibrator action is produced by thek provision between the tubes of a conventional resistance and condenser coupling arrangement- 84, 85, 86 together with a back coupling from the output of tube 90 to the input of tube by virtue of the common cathode resistance 8| which has no by-pass condenser. The forward coupling departs from the conventional only in that the valuesof condenser and resistance 86 are so chosen as to charge condenser 85 to a substantial voltage after a few microseconds.

. Assuming that the initial condition of the multi-vibrator Bil-E@ is one wherein tube 85 is carrying a comparatively high current, thus making its plate far less positive than the potential of the plate supply, such reduction of positive plate potential on tube 80 will tend to apply and maintain a high negative potential upon the control grld of via coupling condenser 85. The capacity of condenser 85 and the resistance of leak 86 are preferably such that after 90 to 150 microseconds the negative signal on the grid of ,90 would be attenuated sufficiently to permit this tube to draw current, thus spontaneously trans-- ferring the multi-vibrator to the second condition in which tube Si! carries substantial current, while tube B0 is biased below cut-off. However, before such transfer can take place spontaneously, a

8| shifts the potential of both cathodes positively.

This shift has the effect of making the control gridoftube 8f!v more negative in relation to its 9 cathode, bringing the grid potential ioftub'e .80 below cut-oli'.

.Although the. brief Asignal vwhich rendered .the grid of tube 80 sharply .negative lvanishes almost as soon as it arrives, the. `multi-vibrator will nevertheless remain in .the condition to which it has thereby been transferred, namely, thecondition wherev tube SG is carrying substantial current and tube 80 is cutoi'. The .reason for'this is that the increased positive potential Von the plate of tube 80 continues to act through condenser 85 to hold the grid of tube 90 sufIiciently positive to render the tube highly conducting while, on the other hand', the high conductance. of 'tube 90 causes such a drop in cathode. resistor 8l. as to maintain tube 80 below cut-olf.

The action of the multi-vibrator isv illustrated in detail in curves B, C Yand, D sof .'Fig. 5.. Curve B is a heavy line representing the. potential on the control grid of tube 80., curve C is a light line representing the common potential .of the cathodes of the'two tubes 80. andy 9,0., and curvev D a dash line representing the potential of the control grid of tube 90. The curves B and D which represent the potentials of .the two. control grids are plotted with respect to the zero line g', while the curve C representing the potentials of the two cathodes is plotted with respect to a different zero line 07s.

These two diiierent zero axes are used for convenience in order to make 'theI point where a vacuum tube commences. to carry .currentfreadily apparent from the inspection of the curves. The two zero axes ligand; Eik are-displaced by a voltage corresponding to the negative bias required for cut-oir for eitherl of the tubes. Intersections of the grid curves, such as curve B or curve D, with the cathode. curve C. represent the points where the corresponding tube commences Vto carry current.

Points a, b on curve D, represent the transition from the initial condition to .the secondary condition under control of the brief needle-like pulse. The portion b1, c of the. curve D represents the gradual vanishing through leak 86 of the positive charge. on tube 90 as a result of the charging up of condenser `85. At the same time the potentials of the two cathodes will also fall as shown by portion q, 1' of curve C.

At point r the cathodes just Vreached such potential that tube 80 can carry current which is indicated by the fact. that curve C intersects curve B. The grid of tube 9i! becomes. more negative, whereupon .tube `8i! will carrystillmore current etc.

The abrupt drop of tube Buitoni point c. to` point d represents the spontaneous shift of the multivibrator from its second condition to its initial condition.

The potential shift of the grid .from e to f is caused by the vertical portion of the rectangular wave form applied to the grid of tube 8D. AThis grid shift has no immediate effect. l

The next shift from initial condition to the second condition is represented by d", b" produced by the next needle-like impulse.

The portion c",` d of the curve represents the next spontaneous restoration to the initial condition.

It will be. noted that the time interval between the triggered shift a, b. and the spontaneous restoration c, d` is considerablyushorter than the corresponding interval between 'the triggered shift a, b and the restoration c", d". -This results from the negative bias applied to the j;

'grid "of tube v84? dur-ing the interval p"","". No

such inequality of successive 'intervals occurs when the rsignals onthe grid of tube -B rare briefly symmetrical `dueto a ybalanced condition ofA tubes E2, 63.

The output from the plate circuit of tube is shown Aby curve E of Fig. 5, this curve representing the voltage across` load resistor 96. The output coupling condenser is made small and has connected to it an output resistor 91 of low enough value so that vthe, time constant of 95 and 97, together is exceedingly short, i. e. of the order of one or two microseconds. The circuit. 95, 91 acts, therefore, as a dilerentiating circuit so Ythat the output voltage represents the rst derivative oi the wave form E, consisting of a seriesof needle sharp pulses alternately reversed in polarity, as shown in curve F of Fig. 5. The negativ-e .ones-of these needle like pulses (shown extending upwardly in curveF) are uniformly spaced since their time 4position corresponds to the needle-like pulses of curve B.. The positive one of these pulses, however, (shown extending downward in curve F) are .non-uniformly spaced depending upon the. value of .the applied speech signal. Thus, in the left .hand portion of curve F, corresponding to an interval of maximum positive potential of theA applied signal, the non-- uniformity of spacing of these positive output pulses is greatest. In. the right hand half of curve F corresponding to an `interval .of maximum of negative potential ofl the applied signal, the. non-uniformity of spacing vO f' these negativ output pulses is zero. Y

The output of converter .I-tl .includes a series of alternately positive and negative needle sharp pulses, the positiveones lof which have a. nonuniformity ofspacing which proportional to the voltage of source 8.. These positive pulses slightly vary-in amplitude asv well .as in time position, although lthis effect can be minimized by proper choice of operating adjustments of the multi-vibrator circuit.

Referring again to Fig.v l, the output from the converter is Apassed through a two-level limiter II. This two level limiter is adjusted to trans-.- mit only a part of the positive portions of the output oiv converter In and `none of the negative pulses. After passing 'through the limiter II the signals have the form of a series of needle sharp pulses of equal amplitude but non-uniform time spacing. These pulses are then applied to a combiner unit I3. which serves to combine them with the :signals .from a speechisource I2 carrying unimportant or misleading signals. The combiner I3 may be an ordinary modulator in which amplitudes of the. needle sharp pulses are modulated in accordance with the speech signals. Or, as an alternative, a linear amplifier may be used in which the .amplitudes of the sharp pulses are added tothe amplitudes of the speech signal. The output. o'f combiner I3 may thus have the .form fof a Yseries of sharp pulses only yor the form of 'a speech wave with substantially larger pulses superposed thereon. In the latter case the next unit must be biased so as to respond only to the needle-like portions of the signal but not to the speech portions alone.

The combined signals from unit I3 are applied to an .oscillator l5 which isnormally biased to prevent oscillations but is .adapted to be brought into .the oscillating .region by the sharp peaks of the signal from I3.. Since the sharp peaks from I3 are not `only displaced in time (as at the output. o unit Il) but are also of variable peak amplitude (as a result -of multiplication by speech signals or superposed upon speech signals), the output from oscillator I will consist of short trains of carrier frequency nonuniformly spaced in time and of non-uniform amplitude. The amplitude variations of the carrier trains correspond tothe signals from I2 and the time variations to signals from source 8.

Transmission of signal pulses in the presence of jamming pulses When the jamming station, Fig. 2, is in operation, then sharp pulses at the rate of 1200 per second will be produced by units 4I, 42, 43, 45 in the same way as by units I, 2, 3, 4, 5.V These pulses modulate or key the oscillator 40 so that brief trains of carrier frequency are delivered from this oscillator over network 46 (if one is provided) to antenna 48. It will be recalled that the jamming station of the simple system A includes only the units 4I, 42, 43, 45, 4U, 48, and possibly 4S 'and 47, the additional elements such as 49, 53 and those within the dot-dash lines of Fig. 2 being disregarded for the moment.

It is desirable that signaling pulses which are simultaneously transmitted with jamming pulses be in xed time relation thereto- To accomplish this, the jam/no jam switch, Fig; 1, is thrown to the right, closing a connection from antenna I8 via network I6 and knife blade y`s5 and s to the blocking circuit 20. From circuit 20, the received jamming signals pass to unit 2| whose intermediate frequency output" goes -to a high threshold circuit 30 which -is adjusted to pass only signals exceeding a predetermined amplitude level, i. e., the strong signals from the jamming station, and block all weaker signals.

The output of circuit 30 is detected and smoothed in detector 3i whose output consists of a series of sharp pulses coresponding to the output of circuit 45. Assuming that the jamming signals occur at the same frequency as the pulses used in signal transmission, the output of second detector 3| will have a frequency of 12,000 per second. These pulses are passed through a sharply selective circuit 32 which selects the purely sinusoidal 12 kc. component. The circuit 32 should be designed to have as low a decrement as possible, if necessary by providing regeneration. Thus, a flywheel action will be provided which will prevent the system from being thrown out of synchronism by occasional crashes of static which may be high enough to pass the threshold of circuit 30.

The 12 kc. sine wave from circuit 32 is applied to both phase Shifters 35 and 33. The latter has no 4effect at the present time since its output extends through 34 to blocking circuit 22 of the receiving portion of the station which at the moment is disconnected in knife blades tsl, ts of switch TS. The 12 kc. waves passing through phase shifter 36, however, are applied to a half- Wave rectifier 3l yielding waves such as were obtained from 2, and as are shown in the right hand side of curve J of Fig. 6.

From half-wave rectifier 3l the waves pass over knife blades ysI, 7's2, to differentiating circuit 3 which by differentiation produces waves such as shown in the right hand side of curve K. Fig. 6. The portion lying between levels .r and y of this curve passes through two-level limiter 4 and, after further differentiation in circuit 5, the waves resemble the curve M in Fig. 6 which, it will be recalled. correspond to the pulses olitained when full wave rectifier 2 was connected.

The pulses in the output of differentiating circuit 5 have a xed timing with respect to the jamming signals received instead of a timing related to the local sine wave source I.

The reduction of a rectangular wave form from vthese sharp pulses in multi-vibrator 6 and limiter 'I occurs just as in the previous case. The response of converter Il] to the three inputs from 5, 9 and 8, is the same as before and all the succeeding units Il, I2,'I3, I5, I6 and I9 operate as before.

One other important change in operation should be noted. Knife blades y's3, y's4 close the circuit from the output of differentiating circuit 3 through two-level limiter I9, knife blades ts, ts4, y`s3, 7s4 to the blocking circuit 20. The two level limiter I9 is arranged to pass only so much of the signal from 3 as lies between the thresholds n and m. Since the output of circuit 3 has the form shown at the right of curve K in Fig. 6, theV portion passed by the two-level limiter I9 will have the form of curve L. The wave L blocks circuit 20 for the short intervals during which oscillator I 5 of the transmitter part of the station is operative to transmit signals. Theoretically. if conjugate network I6 provided perfect compensation between the receiver and transmittel', such blocking would be unnecessary; in practice it is required. Using the conjugate network I6, the blocking circuit 20 need only withstand the modulated voltages which may pass through network I 6 on account of its'ime perfect balance. If the network I3 is eliminated, then the blocking circuit 20 must withstand the full voltage from oscillator I5. The blocking may be made more perfect by providing two or more blocked stages in circuit 20.

Reception of vsignals in the presence or absence of jamming When the talk/listen switch TS is down in the listening position, antenna I8 is connected through network I6 and knife blades ts5, ts to the unit 20 regardless of the position of switch JS. Limiter I9 is now disconnected from the blocking circuit 2D at knife blades ts3, ts4 and the oscillator I5 is disconnected from the network` I6 at blades tsl, ts2. 'Ihe station will, therefore, receive regardless of the position of the switch JS.

The operation is as follows: Incoming signals pass from antenna i8 through network i 6 andl blocking circuit 20 which is not blocked locally since the circuit to its upper terminals is opened at ts3, ts4. From circuit 20 the signals pass to unit 2I where they are converted to intermediate frequency. From 1 unit 2| the signals pass through blades tsl, ts8 to externally controlled co blocking circuit 22. This circuit normally blocks the signals, but it is adapted to pass them when an unblocking potential is applied to its upper terminals. In a manner to be more fully described, the units 30, 3i, 32, 33 and 34 serve to 65 apply such an unblocking potential for a brief period which is so timed as to include the instant of arrival of the desired signal pulse.

Each signal pulse from the unit 2l passes through blocking circuit 22 and is then applied 70 to the two detectors 21 and 35. The detector 2'! detects' short L. F. trains corresponding to the signal pulses, and its output is sm-oothed by a low pass filter which not only limits the intermediate frequency ripple from the detector out- Ys put but removes also the 12 kc. ripple produced by' the separate pulses. Only the slower amplitude variations which correspond tothe. amplitude modulations of the short pulses. by source'li of the signal transmitting station will be picked up in the earphones 2S. This may be 'a meaningless masking signal or an intelligible mask:- ing message but concerning routine matters.

The detector 35 also detects the short trains of intermediate frequency waves to yield sharp pulses, but its ripple smo-othing arrangement vis arranged to eliminate only intermediate fre quency ripple and retain the fundamental I2 kc; frequency of the individual pulses and as many harmonics as possible. The output #from the detector and smother unit 35 will have substan L tially the original needle sharp wave form of the pulses from the transmitting station. These sharp pulses are then passed through a twolevel limiter 23 to eliminate the rounding corners and amplitude modulation, and to accentuate. the harmonics.

From limiter 23 the waves, which are now: characterized by variable time position but constant amplitude, pass through a harmonic se lector 24 which is tuned to the 11th harmonic of half the pulse frequency, i. e., 6 kc. The harmonic selector may be tuned, but not so sharply as to clip the sider band representing speech. For low quality speech of 60-3000 c. p. s., a simple tuned circuit' of modernate Q may be used, especially when a harmonic as high as the 11th is selected. For higher quality speech, however, or when a lower harmonic must be selected.a band pass lter should replace the harmonic selector 24.

As described in the copen'd'ing application of Emile Labin abovereferred to, the amplitude of any odd harmonic of half the pulse frequency will be proportional to the degree of non-uniformity in the time spacing of the pulses, and thus to the amplitude of the secret signal 'from source 8. The proportionality is truly linear only When the maximum positional non-uniformity of the pulses is very small and when the harmonic selected is not too high. Greater amplitude variations can be obtained with the higher harmonics. Under most practical conditions, ifV the amount of variation in time position is kept small in comparison with the total cycle time, i. e., of the order of 2% thereof, it is desirable to use a rather high odd harmonic such as the 11th. In most cases the primary limitation in the selection of the harmonic is the impossibility of maintaining pulses needle sharp, whereby the higher harmonics (say the 15th and higher) tend to fall half in amplitude. The preferableupper limit of' harmonic selector 24 is, therefore, a 6 kc. wave whose amplitude varies in accordance with the secret speech from source -3 of the transmitting station. The output of this harmonicselector is then applied to a detector 25 which is provided with means for smoothing out the 66 kc. ripple. The output of detector 24 is made audible in earphones 26 which reproduce the secret speed signals from source 8 at the transmitting station.

. It has been mentioned that units 3Q, 3l, 32, 33 and 34 serve to provide a cle-blocking signal for circuit 22 at brief time intervals corresponding: to the desired incoming pulses. The manner in which this is done depends somewhat upon whether the signals are received in the presence or absence of jamming.

Let us assume rst that the TS switch is. inv the listening or down position, that the JS switch 'is'.closed. jamming pulses fare receiyed. Threshold circuit 3f!v just passesthe sig@- nal puisesand occasionally a small amount of sta-tic and. other disturbances which have no harmfuleffect, llromfthe high threshold circuit 30' the pulses. vand occasional static pass through the second detector` 31. and Ythence through the sharplyrselected circuit 32 which by its iiywheel actionprevents the static from having any substantial desynchronizing eiect. The 12 kc; sine waves from 32= which have a definite time relation; to. the incoming signal pulses are then passed. through phase shifter 33 and then through the; two-level limiter 34. The levels of limiter 34 are chosen veryl close. to. each other and to the peak level of the sine Waves which will, there-1 fore, be close. to their` apices. .Only thosepor`-V tions of the waves pass through limiter 34. which are iapproxixnately rectangularand of compara.- tively short; dination,l say one-tenth of a cycle long. Then-phase shifter 33 isv adjusted to time these brief! de-blocking; pulses. of substantially rectangular wave. form so as to unblock circuit 2.2, just beforey the arrival of each one of the desiredipulsesffrom unit' 2|' and to maintain this circuit unblocked untily slightly after each such pulse has ceased.

Whenjamming pulses. are. received With the signal pulsesthe operation is: essentially the same except that threshold circuit13!) passes only the strongerjamming pulses butinot the weaker sig-v naling pulses, the, phase shifter being' corre spondingly'readjusted.. Since; the jamming pulses have a definite. time relation tothe signal pulses, they 'may be; used Successiully for` controlling. the fle-blocking.

`VARIA'IIONS OF SYSTEM A 'System B with: monitoring at jamming station In the 'simpfe System' A, the jamming station hadno means for ascertaining whether its wave length was such as'to cause themo'st interference with the-enemys communications nor whether the friendly Vsigna-ls were properly synchronized with its jannni'ng pulses. To provide for such 'supervision the jamming Vstation may be modified to include the complete circuit of Fig. 2. The system including suchmodied jamming station, together with a plurality of combined signal transmitting and receiving stations according to Fig. l, will bereferred to as System B. To facilitate comparison, the same units digits are used to designate corresponding 'elements of Figs.' 1 andi-2'. Y' Y A l The'addltional equipment'in Fig. 2y comprises two receivers 5l and 55 arranged to apply their outputs `to. two cathoderayf tubes 52 and 51, respectively. `Means are provided for cyclically tuning the/second receiver 55 over a Wave band wide enough'to cover all or part of the frequency spectrum likely to; be used by the enemy.: Accessoryfeqnipment HSI.,j 1.62, 54 is `needed ,for rendering the cyclically tuned receiver 55 insensitive to ltheriendly signal pulses, and equipment 49, 50 for rendering both receivers A5| and 55 insensitive to thepulses from the jamming transmission station. Suitabley sweep circuit generators 53 and 58 are provided for the tubes 52 and`5l'.

The method of operation o f the jamming station-is the ysame as beforel as far as transmission of'jamm'ing pulsesy is concerned. The monitoring operation' is as'follows:

The enemy .signals and friendly signals are both received on vantenna Vl'and transmitted over network 46Vto the blocking circuit 50. This circuit normally passes signals but is arranged Ato block transmission of all signals for a. brief moment when jamming trains and carrier fre-- quency are emitted from oscillator 46. Such momentary blocking pulsesv are delivered over two-level limiter 49, which receives part of the output from dierentiating circuit 43, in the same manner as the corresponding blocking circuit is controled by two-level limiter I9 in Fig. 1.

Except for brief blocked intervals when jamming trains are sent out by the station itself, the incoming enemy and friendly signals pass through circuit 50 to the receiver 5|, which is manually turned to the frequency of the friendly stations. The output of this receiver is applied to the vertical deiiecting plates of cathode ray tube 52. The horizontal sweep wave generator 53 is triggered by the needle sharp waves from differentiating circuit 45 giving the saw tooth waves a fixed time relation with respect to the jamming waves sent out. The time position of the friendly transmission waves received by receiver 5| will, therefore, be shown on the screen of cathode ray tube 52 in relation to the timing of the jamming waves from the jamming transmission station itself.

The balance of the energy from blocking circuit 50 is applied through a circuit 54 to receiver 55 which 1s automatically cyclically tuned by motor 56. This motor 56 drives also a rotary gen-4 erator adapted to produce saw tooth wave forms suitable for a sweep circuit. Such'rctary gen-- erator may be a magnetic generator with speciallyv formed pole pieces, or it may consist of a rotating potentiometer and a battery connected thereto.`l

The outputs from receiver 55 and from rotary generator 58 are applied, respectively, to the horizontal and lvertical deflection platesof cathode ray tube 51. The generator 58 is synchronized with the tuning receiver 55 in suchA a way that for each different frequency forwhich. receiver 55- is tuned, the generator 58 will give a different out-,- put potential, the potential preferably varying linearly with the frequency to which the receiver 55 is tuned. The screen of cathode ray tube 51 will, therefore, show a vertical line produced by the varying sweep voltages from generator 58v horizontally deiiected at the point corresponding to the frequency of the enemys communications.

If the enemy has two or more frequencies-of communication, a corresponding number of horizontal deflections will appear upon the screen.

A small amount of energy from the oscillator 40 of the jamming station may be transmitted over leads L54 and key 66 to the input of receiver 55, so that a horizontal deflectionwill also appear on the screen of tube 51 indicating the frequency of oscillator 40 provided at the transmitting station. If the oscillator 40 is accurately tuned to the same frequency as the enemys communications, the deiiection occurring in responseft'o the depression of the key 66 would fall exactly on top of the deflection representing the lenemys communication frequency.

In order to prevent the receiver 55 from re-y sponding to energy from the friendly signal transmitting station which is assumed to be operating in synchronism with the jamming pulse transmission station, the apparatus |60, |6l and |62 may be provided periodically to transmit blocking potentials to the circuit 54. The length of' ap.- plication of blocking potentials maybe manually controlled to occur in a predetermined timed re- 16 lation to the instant of transmission of pulses from the jamming transmission station.

Units |6| and |62 of this equipment operate as indicated in the left hand portion of curve J, Fig. 6, the rectifier unit 6| serving to produce half sine waves, and the two-level limiter |62 to pass only those portions of these half sine waves which lie between levels g and h. The blocking arrangement 54 is normally adjusted to block passage of current from to 55 but is unblocked by the substantially rectangular waves which pass through limiter 62. The poi'- tions lying between the limits g and h of curve J lll most of the cycle time so that the circuit 54 will be unblocked by these impulses for the greater part of the cycle and will only be returned to blocking position for a brief interval in each cycle. By adjustment of phase shifter' |60 the time position of the brief blocking interval can be made to cover the instant when pulses are transmitted by the friendly signal transmitting equipment.

It will be seen, therefore, that the blocking circuit 50 serves to protect receiver 55 from the signals emitted by the local oscillator 40 of the jammingV signal transmitter, while blocking the circuit 54 protects the receiver 55 from the friendly remote transmitter such as shown in Fig. 1. The only signals which will affect receiver will be those of the enemys communications, and only the frequencies of the enemys communications will be indicated on the screen of tube 51 except when the frequency of the oscillator 40 is shown upon the depression of key |66.

Ordinarily in operating System A or System B, it is assumed that the jamming station will be found to have the same frequency as the enemys communications. The friendly signal transmitting stations, Fig. l, will also be tuned to this-frequency or to a frequency very close thereto. If the enemy varies his frequency so as to escape, the jamming signal will at once show up on the screen of cathode tube 5l. If,

- on the other hand, the transmitter of a friendly station, Fig. 1, gets out of synchronism with the jamming transmission station, e. g. by being slightly mistuned, this fact will be indicatedy upon the screen of cathode ray tube 52.

SYSTEM C Fig. 4 represents a slightly modified form of combined signal transmitting and receiving station which may be substituted for that of Fig. 1. When such Vmodified system is employed, the jamming station should be modified to emit pulses at the rate of 6000 per second or a submultiple thereof rather than 12,000 per second. In describing System C it will be assumed that source 4| of the jamming station of Fig. 2 operates at 3 kc. instead of 6 kc., thus causing the emitted pulses to occur at the rate of 6000 per second..

The combined signal transmitting and receiving station of Fig. 4 very closely resembles that of Fig. 1, the like numbered components being identical. The chief differences are:

(a) Full-wave receiver 2 is connected through knife blades fsl, 7's2, and the half-wave rectifier 31 is omitted.

(b) The feed from 3| to phase shifter 36 now passes through a modified selective circuit 32 which is tuned to 6 kc. instead of 12 kc. like circuit 32. The 12 kc. circuit 32 is retained but feeds only the phase shifter 33 and not 36.

17 '.(c) The .rectangular input .pulses .of .converter I 0 are obtained by doubly .limiting ysine fwaves fromsource I and notas in Fig. .-1-by operating 'a doubly triggeredmulti-vibrator 6 fed=from the output -of f5. The two-level limiter 'i is, .therefore, .set to pass only the portionfof .the `sine wave adjacent the `zero axis, producing -.anal'` most rectangular wave .form 1With -the ipeaks and valleys of .substantially .equal length.

The operation of Fig. 4 is similar to `Fig. .1 and need .be vonly lbriefly described. The :sharp pulses for application to converter vIi! 4arewobtained as before,except thatthelimiter li-is omitted. The-rectangular wavesuvhich form another input'to'converter I0 Yare obtained-,by the limiting ofthe Waves .from source'l inlimiter l. The time position of the rectangular .Waves `Withrespectvto the :needle sharp input Waves .is `.regulatedby phase shifter Bti-.instead of by-anarticial line 9, as in Fig. 1.

Thefunctioning of converter IQ and .of Athe succeedingfequipment Il, I2, I5, .16, I and I3 is theisame .as before.

When vthe transmitting v.equipment .of Figi is operated during jamming, v the incoming jamming fpulses :received by -18 .pass :through 16, -thence .through 7295 and 7's6 .to 2G, .and thence `through 21530 and v3|, exactlyas before,-.except 'thatspulses occur lwith a frequency of #6000 -per second. From the :output Pof ldetector f3I, the

pulses pass through .a .sharply `selective circuit 32 V.tuned to .6kc..and thence through'a phase shifter 36 tothe knife blades jslgg'sZ.

.The Waves thus :received via=36 'fromi32' are like ithe *Waves which Vwould `be received from source I in the absence of jamming and, there- .fore the'rest -of .the ftransmitteroperates without jamming.

in Ith'eicase of recepton'lwithout jammingfFig.

4. operates like-Fig. -l, the x12 kc. .circuit .32 `=re 1 .sponding to thepulses and*un-blockingvcircuitfn so as ftogpass these pulses.

In'lthe -case fof reception lduring ijamming, YY*the high .threshold circuit '35 is adjusted :to #pass .only'nthe :jamming pulses as. in Fig. 1. 'tection .in -z3I vthe n6 fkc. Y'pulses tgo through-cir- .zeuit .32. Since, however, l'the fsharp pulses are every rich insharmonics fa fsubstantially 12 kc. component Will exist and pass :through vcircuit limiterf;34 .to tun-'block 1circuiti22 vat :the .required .'tim'es. The-remainder -of the receiving iopera- 'tion'sfarelthe sametasrin Fig. i1.

`"Although yin fSystems 2A, FB, and 2C, "the '.'jamf- -ming Atransmission A.station lwas 'ass'umed to 'be .very .low .audible .frequency -for the :jamming l pulses. The adjustments of .elements32 and32' iin ligs.l1andf.4, need not'be altered since even ata fundamental frequency :of v.375 kc. .or'lower therefwill.beenough 6 `kc.and:12-kc..components IAfter deto actuate these circuits. Such modif-led system willbereferred to as A', B and C.

.Astill -more `desirable effect is produced by modulating the jamming pulses at one or more audio frequencies. For example, the jamming pulses from differentiating circuit L15 (Fig. 2) may be modulated by audio signals from a source |16 in am-odulator V55 before-being used to modulate oscillator fill.

The jamming station may then-be combined with Fig. 1, and operated with'switch JS in ythe nojarn position illustrated. The unused .por- .tionsof the station could be omitted, or all the equipment shown in ./Fig. 1 could be retained -to permit -.the.alternative use of a remote jamming station.

:Iffinstead of the .station equipment of Fig. 1, that of Figifis employedV as the jamming stationl suitable .jamming noises are supplied to.source 8. The switch JS is in the .no-jam position so as `to make .the -station independent -of outside niluences.

The monitoring equipment shown-within 4the dot-dash lines of Fig. 2 could be connected to the .circuit of Fig. 4 when the latter is used as a vjammingtransmissionstation, the leads LSI, L53, L55 and L60 being connected to the correspondingly labeled terminals in Fig. 4. With the monitoring equipment added to Fig. 4, System E is changed into a .Very flexible system.

In all the` embodiments, when simultaneous message and jamming signal transmission was desired, .the signal .transmitting station syn- .chronizedtself with .the jamming. station. For agreat .many purposes it is preferable to reverse the procedure and arrange Ythat the jamming pulse transmitting .station .should synchronize itself with the .message `signal `transmitting station, the latter being .then operated `under the controlof its local source. For this purpose, the equipment of Fig..1 is usedas .the jammingsignal transmitting station .3 with .the switch JS in the jammed position, Asuitable .audio frequencies being applied Eto source I2 to produce the `jamming signals.

4A plurality of combined signal .transmitting and receiving stations like I'Fig. f1 may -be operated with their switches JS .inthe ,no-jam position at all times. vWhen one ofthese stations is transmitting .it will be .under thecontrol of itslocal oscillator I. A jamming station likeFig. 1 will haveits switchv JS in the jam position-and-will synchronize itself with the message signalsreceived.from another station.

VIf it isdesired to .employ the circuitsof Fig. .4 in the last mentioned system, thejamming station should be arranged to operatefat .two ormore times :the 4.frequency v:of l`the `signal Vtransmitting .and.:receivingfstations. Circuit .-32 r of the jam'- .mingistation should'thensbe-adjusted `to 12jkc., .andthe converter-.unit Ill-should respond to sharp pulses-.occurring at thefratepf 24.00y perfsecond. l:".Fhe-sine lwa-veisource rI 'maybe `completely v elimi- .natedsince the jammingstation =Willabeused only withrthe switchJSthroWn to' the? right. Likewise the elements :32533, 34,2-2'2,f.35,r23,f.24, :25,16, 21, wand/2 9; may be. omitted.`

This system may be ampliiiedeby addingtoathe .jamming station, ine., .to the station which has beenffstepped up infrequency fand has 'had the .receiving portions deleted, themonitoring .equip- .mentfof Fig. f2.

Alternatively, l. the monitoring equipment may ..be.applied :'.to the :several combined signal. tran's- :mitting and 'receiving stations.

In any of the above systems the jamming signals may be modulated with apparently bona fide masking messages instead of with random noises in order still further to confuse the enemy and still further conceal that speech signals are being simultaneously transmitted by one of the signal transmitting stations.

If the jamming station and the signal transmitting stations are so far apart that the time of propagation of radio waves from one to the other is substantial, a third station disposed at unequal distance from the jamming and transmitting stations may simultaneously receive the jamming and signaling pulses. Ir the stations operate at a pulse frequency of 12,000 per second, this could happen only if the distance between the stations corresponded to a time interval of at least the same order of magnitude. No such diiculties could occur at the contemplated pulse frequencies. However, if higher quality speech transmission is desired, this limitation of the system should be borne in mind.

We claim:

1. The method of masking messages comprising the following steps: producing impulses, modulating at least some of said impulses in accordance with one system of modulation by one message, and modulating at least some of said impulses in accordance with a different system of modulation by a second mesage.

2. The method of masking messages comprising the following steps: modulating impulses by one message in accordance with one system of modulation, and modulating at least some of the modulated impulses by the second message in accordance with a different system of modulation.

3. In combination, means for producing impulses, a i'lrst source of signals, means for modulating at least some of said impulses in accordance with one system of modulation by the signals from said rst source, a second source of signals, and means for modulating at least some of said impulses in accordance with a different system of modulation by the signals from said second source.

4. In a secrecy transmission system, means for producing impulses, a rst source of signals, means for modulating at least some of said impulses in accordance with one system of modulation by the signals from said rst source, a second source of signals, means for modulating at least some of said impulses in accordance with a different system of modulation by the signals from said second source, means for combining the outputs of both said means for modulating, and means for transmitting the combined modulated impulses.

5. In a transmitter, means for producing impulses, a first source of signals, means for modulating at least some of said impulses in accordance with one system of modulation by the signals from said first source, a second source of signals, means for modulating at least some of said impulses in accordance with a diierent system of modulation by the signals from said second source, a source of carrier frequency, and means for modulating the carrier by all said modulated signals.

6. In a transmitter, means for producing impulses, a first source of signals, means for modulating at least some of said impulses in accordance with one system of modulation by the signals from said first source, a second source of signals, and means for modulating at least some of the modulated impulses in accordance with 20 a different system of modulation byl the signals from said second source.

7. In a transmitter, means for producing a series of impulses spaced apart in predetermined relationship in time, a rst source of signals, means for time modulating at least some of said impulses to vary their positions in accordance with signals from said rst source, and means for amplitude modulating at least some of said impulses.

l8. In a transmission system, means for producing a series of impulses spaced apart in predetermined relationship in time, a first source' signal energy, modulation means for time modulating said impulses to vary their positions in accordance with signals from said rst source, a second source of signals, and means for amplitude modulating said time displaced impulses in accordance with signals from said second source of signals. Y

9. In a transmission system according to claim 8, further comprising an amplitude limiter to limit the amplitude of said time modulated impulses to Vsubstantially the same level before modulation by signals from said second source.

10. In a receiver for time and amplitude modulated brief trains of carrier signal Waves, a iirst detector responsive to the amplitude variations of the signals, a second detector responsive to the signal waves, a limiter connected with the second detector to eliminate amplitude modulation from the detected signals, and a harmonic selector connected with said limiter and to produce signals having an amplitude envelope corresponding to the time modulation of the signal waves.

11. The receiver according to claim 10, an antenna, a blocking circuit connecting said detectors with the antenna, said circuit being adapted to pass signals of a certain range of frequencies except when blocking signals are applied thereto, and means for periodically applying blocking signals to the blocking circuit.

12. In a receiver for time and amplitude modulated brief trains of carrier signal waves, an antenna, a rst blocking circuit connected therewith and adapted to pass signals of a certain range frequency except when blocking signals are applied thereto, a frequency changer connected with the blocking circuit, a second blocking circuit connected with the frequency changer and adapted to block all signals except when a signal potential of predetermined magnitude is Aapplied thereto, means operable at will at the reeciver for periodically applying said blocking signal to the first blocking circuit and the said signal potential to the second blocking circuit, a first detector responsive to the amplitude variations of the signals connected with the second blocking circuit, a second detector responsive to the signal waves connected in multiple with the iirst detector, a two-level amplitude limiter connected with the second detector to eliminate amplitude and noise modulation from the detected signals, a harmonic selector connected with said limiter and adapted to produce signals having an amplitude envelope corresponding to the time modulation of the signal waves, and a detector for the last-mentioned signals.

13. In a receiver for time and amplitude modulated signals, an antenna, a low frequency lter circuit connected with the antenna, a high threshold circuit for passing only that part of the output of said lter circuit which exceeds a predetermined amplitude level, means for deagregara 2l tecting 'and smoothing the `A'output ef -the 'last mentioned circuit, 1a circuit lfor selecting alsinuvscildal vcomponent of the 'output A'ofi the last means, y'a detector for Tall signals, afsecond detector lfor 'the l amplitude modulation of vthe signalsga circuit forbl'ocking both detectorsexcept -vvhen predetermined 'potentials fare applied thereto, and two connections for "applying-"such petenti'als 'one connected 'with the "output dif the l sinusoidal component selecting circuit I`and 1 the-otherwith the output ofltlienltereircuit.

I14k-:In combination, a radio transmitter, a 4radio lr-"eceiver, foutput 4terminals for the trans- "niitte'i land input f terminals for 'the lr'eceiver, a'n 'antenrapa conjugate coupling network coupled withf-the antenna and-comprising also terminals `for Athe transmitter Aand terminals .for 'the 'ire- 'ceive'r Twho'se conjugacy is ipreserved, vand'a ltwopdsitio'n l talk-listen vsvv-itch having contacts ASconnecting in 'one 'position only .the terminals 'of the lltransmitter Sand 'in the fsecond position ronly ythose 'of the receiver' with the vcorrespond- Eiirigltermin'als inthe network.

515. `VInla communication system,'m`eansfor pro- "ducing'a series of-impulses spaced apart in predetermined relationship .in time, a `iirst source signalenergy, modulation means for time modulating said impulses fto vary their positions in fac'cordance withLsignals-from said rst source,'a

second source of signals, means foramplitude :w

modulatingsaid time displaced impulses inac- Icordance with zsignals .from vsaid Ssecon'd source of signals, means `for receiving .said impulses modulated by signalsv from said rst'and second lsources, means for detecting .said receivedsig- Pnals-to reproduce signals from saidsecondsource, "-'se'lection and limiting means for reproducing f'rsaid time modulated impulses independently of fsaid amplitude modulation, and means for repro- 'du'cingfsignals from said rstsource under con- 'Ipuls'esindependently of 'said amplitudemodulation, and -means for reproducing -signals Vfrom hvsaid irstsource vunder control'ofl said reproduced impulses.

' 17. In combination, a radio receiver,me`ans 'for Ycyclically Atuning the receiver through 'a vrrange of frequencies, means for visually indicat- 'ing the frequency of the signals received, a source v`di jamming signals, means including said re- 'fceiver forvisually indicating the relationship between the'requencies of the jamming and other.

"signals received, anda second sourceof jamming -si'gnals controlling said receiver.

18. 'A communication system for pulse trans- 'mission comprising means for producing a series 'of impulses-spaced apart in predetermined rela- `tionship in time, a first source signal energy, vmodulation means for tirnemodulating'said im- YYYpulsesto Vary their'positions-in accordance with signals from said first source, a second source "ofisig'nals, means for amplitudemodulatingsaid" 'time ldispl'a'.'ced fimpulses Lin accordance with `-signals from said second .source of signals, 15a -jamming station 'for producing'jamming impulses, means for adjusting the vfrequency of 'said jamming station, means for indicating the-normal time spacing of said impulses, andthe time 'position of said jamming pulses with respect to vsaid impulses. Y

19. -A communicationsystem for pulse translmission comprisingv means for. producing a-series of impulses Tspacediapart in 7.predeterminedrelationship in Stime, a 4irst source signal energy. modulationme'ans for time modulating said im'- pulses tovary their lpositions lin accordance with signals 'from'said nrs't'source, a second source .of "signals, means -for amplitude modulating .s'aid ltime `displaced lim'pulses in accordance v'with signals "from said' second .source of signals, 'La fi'lrst station comprising .means Vfor emitting jamming signals, a second station havingiradia't- `ing 'meansifore'mitting said impulses modulated 'with 'signals lromsaid rst and second sources, 'means forre'ceivingsaid jammingsignalslat said second station, 'means forproducing waves under control. of said received jamming signals, and means undercontrol oisaid produced waves `for operating said impulse 4producing means'where- Iby production of said impulses Will be -synchronize'd with said jamming signals.

'720.In combination, `a 'radio signal receiver,V

means forcyclically tuningthereceiver through `arange of frequencies, a cathoderayltubahav" ing "pairs of vertical andhorizontal xdei'lecting plates-a circuit for applying the `output of rthe receiver to fone pairI of plates, a `sweep circuit generator connected with the other pair 'of plates, lmeans to alter the potential outputof saidgenlerator in relation `to the frequency towhichth'e 'receiveris tuned, whereby the frequency .of .the

radiosignals received will be visually indicated,

la source of jammingsignals, means operable-.at Will for applying jamming signals to the'receiver, 'ablocking circuit connected Withsaid receiver,

the point Vor" application of said jamming-signals being in `the connection between the receiver Tand 'the vblocking circuit, a second source 'f jamming signals, and' means for applyingsignals "freinltlie secon'd 'source zto the blocking "circuit `to -unblock the l. receiver.

2l. -InI combination, .'a receiver of radio signals, means operable iat Willfortuninglsaid receiver,

aica'thode'ray tubeili'aving-'a 'pair o'f horizontal anda pair-of vertical delecting platesfaco'n'- nection from thereceiver to one-pair ofplatesfa sweep,generator-connected to the 'other pairlrof plates, means vior transmitting jamming '1waves, vand a. control circuit for said generator controlled vpair of 'plateameans to alter thepote'ntial out- 'put'of' said generator inrelation to the frequency to which the receiver is tuned, a 'source 'of jamming signals, means for controlling the operation of said receiver by jamming signals, second' receiver, means operable at will fortuning the -second receiver, a second cathode "ray tube having pairs of' Vertical and rotary deiect- "in'g'plates, a 'connectionfrom the second kreceiver vto one pair of the last-mentioned plates, a second 23 sweep circuit generator connected with the second pair of the last-mentioned plates, and a control circuit for the last-mentioned generator controlled by jamming signals. v

23. In combination, a radio signal receiver, means for cyclically tuning the receiver through a range of frequencies, an antenna, a rst blocking circuit connected with the antenna and adapted to pass signals within said range of frequencies, a second blocking circuit through which the first blocking circuit is connected with `the receiver, said second circuit normally blocking passage of all signals, but unblocked when certain signals are applied thereto, a cathode ray tube having pairs of vertical and horizontal deiiecting plates, a circuit for applying the output of the receiver to one pair of plates, a sweep circuit generator connected with the other pair of plates, means to alter the potential output of said generator in relation to the frequency to which the receiver is tuned, a source of signals and means for periodically applying to the second circuit said certain signals from said source.

24. The combination according to claim 23, and means 'for periodically applying signals from said source to the first blocking circuit to block the passage of signals therethrough.

25. In combination, a radio signal receiver, means for cyclically tuning the receiver through a range of frequencies, an antenna, a rst blocking circuit connected with the antenna and adapted to pass only signals within said range of frequencies, a second blocking circuit through which the rst blocking circuit is connected with the receiver, said second circuit normally blocking passage of all signals, but unblocked when certain signals are applied thereto, a cathode ray tube having pairs of vertical and horizontal deflecting plates, a circuit for applying the output of the receiver to one pair of plates, a sweep circuit generator connected with the other pair of plates, means to alter the potential output of said generator in relation to the frequency to which the receiver is tuned, a source of signals, means for directly coupling said antenna with the receiver, means for periodically applying to the second circuit said certain signals from said source, a second receiver tunable at will connected with the first blocking circuit, a second cathode ray tube having pairs of horizontal and vertical deflecting plates, a connection between one pair of plates of the second tube and the second receiver, a second sweep circuit generator connected With the second pair of plates of the second tube means for periodically applying signals from said source to the rst blocking circuit to block the passage of signals therethrough, and means for periodically applying signals from said source to the second sweep circuit generator to control its operation.

26. The method of masking messages comprising the following steps; producing at one station impulses of a certain frequency, modulating at the same station the impulses by jamming signals, transmitting the modulated jamming signals, receiving the signals at a second station and converting them into impulses having a frequency bearing a predetermined relationship to said certain frequency, modulating at the second station at least some of the impulses by secret message signals in accordance with one system of modulation, modulating at the second station at least some of the impulses by a masking message in accordance with a second system of modulation, combining the modulated signals at the second station, and transmitting the combined signals from the second station. p

27. The method of masking messages comprising thesfollowing steps; producing impulses having a predetermined time spacing, modulating the spacing of at least some of the impulses by jamming signals, modulating the amplitude of at least some of the impulses by message signals,

'combining the time and amplitude modulated impulses, and modulating in time spacing at least some of the impulses by other message signals.

28. In communication system, means for producing impulses having a certain time spacing, a first source ofsignals, means for modulating the spacing of at least some of said impulses in accordance with signals from said rst source, a second source of signals, means for the modulating amplitude of at least some of said impulses by signals from the second source, a rst sta tion, means at the station for combining, and transmitting the modulated impulses, a second station, a source of jamming impulses thereat, and means for controlling the time relationship of the transmitting of jamming impulses from the second station to the transmitting of the combined modulated impulses from the kfirst sta tion.

29. The system according to claim 28, and in which the means for controlling the time relationship is at the first station.

30. The system according to claim 28, and in which the means for controlling the time relationship is at the second station.

31. The system according to claim 28 and in which the spacing of the vsignaling and jamming impulses bears the relationship of two-to-one.

32. In a communication system, a first station having a radio transmitter, an antenna, a sine wave source, means for converting the sine waves into a series of impulses having a certain time spacing, a first source of signals, means for modulating the spacing of at least some of the impulses in accordance with signals from the rst source, a second source of signals, means for modulating the amplitude of at least some of the time modulated impulses in accordance with signals of the second source, a generator of a high frequency carrier, means for modulating the carrier by the time and amplitude modulated impulses, a receiver at the station having means for detecting impulses received by the antenna, means in the receiver for reproducing the amplitude modulation of the impulses, means in the receiver for reproducing the time modulation of the impulses, a jamming station comprising means for producing impulses spaced apart in a predetermined relationship to said certain time spacing, a generator of said high frequency carrier at the jamming station, means for transmitting the last-mentioned carrier modulated by the last-mentioned impulses, a first switch yat the rst station having one position for connecting the transmitter and a second position for effectively connecting the receiver with the antenna, a second switch at the first station. means controlled by the second switch and operative in the first position of the first switch for disconnecting the means for converting from the associated sine wave source and connecting it kwith the antenna, and means controlled by the second switch and operable in the second position of the rst switch for applying the jamming signals received by the antenna to the receiver to block its operation.

33. The system according to claim 32, and

means controlled bythe second switch for conquency, a circuit for passing only intermediate" frequency impulses exceeding a predetermined amplitude level, means for detecting and smoothing the output of the last-mentioned circuit, and

a circuit for selecting a sinusoidal component of y v the output of the last-mentioned means havingr Y of`,operation which includes producing pulses a predetermined relationship to the frequency of said sine wave source.

34. The method of secrecy communication comprising the following steps; time modulating Ta second signal, and

zc 5 means for transmitting said plural modulated pulses on saidcarrier Wave.

39. In a pulse modulation communication system, the method of operationwhich includes 1 producing variable energy content and variable repetition rate pulses whose variations in weight l are# in accordance with variations in the signal modulation.

40. In a communication system, the method ."that vary both in energy content and repetition impulses by secret message signals, amplitude' modulating the timev modulated impulses by masking message signals, transmitting the time and amplitude modulated impulses on a carrier,

detecting the time and amplitude modulated impulses, separately reproducing the amplitude'V and the time modulation of the signals, sending jamming signals in synchronism with the transmitting of the time and amplitude modulated signals, and blocking the detecting by the.

message signals, transmitting from a station the l" time and amplitude modulated impulses on a carrier, detecting at a second station the time.

`iiecting plates, a circuit for applying the output jof'fthe receiver to one pair of plates, a sweep circuit generator connected with" the other pair of f plates, and means to` alter the potential output of fsaid generator in relation 'to the frequency to which the receiver is tuned, azblocking circuit for and amplitude modulated impulses, separately 're" producing the amplitude and the time modul'a'l;

tion of the signals, sending from a third station' jamming signals in synchronism with the transmitting of the time and amplitude modulated said pulses vin accordance with a rst signal,

"said receiver, a transmitter and means con- REFERENCES CITED `The following references are of record in the file of this patent: 2

UNITED STATES PATENTS Number Name Date 1,446,433 Schaffer Feb. 20, 1923 1,973,298 Sloggett et al. Sept. 11, 1934 2,151,747 Conrad Mar. 28, 1939 .2,156,374 Crosby May 2, 1939 2,227,108 Roosenstein Dec. 31, 1940 2,262,838 Deloraine et al.' Nov. 18, 1941 V2,266,194 Guanella Dec. 16, 1941 2,279,151 Wallace Apr. 7, 1942 12,381,847 Ullrich Aug. 7, 1945 2,392,546 Peterson Jan. 8, 1946 y FOREIGN PATENTS Number Country Date 541,665 Great Britain Dec. 5, 1941 means for modulating the energy contentof said time modulated pulses in accordance with 

